A high gloss surface is often desired in applications such as photographic papers, high quality copier papers, wrappers for foods, chewing gum, and clothing, and other flexible packaging. The eye-catching appeal of high gloss coatings is considered a selling point in the retail trade.
Likewise, good see-through transparency is important in flexible packages made from blown or cast films such as clothing wrappers, permitting a buyer to examine the contents of a package without the necessity of opening the package. This is also important in certain extrusion coating applications such as document laminations.
U.S. Pat. Nos. 5,268,230 and 5,350,476, issued to one of the present inventors, disclose a coating composition and process of coating therewith, producing a single, dual purpose, high gloss polyethylene extrusion coating with a gloss comparable to or even greater than that exhibited by polypropylenes. By "dual purpose" is meant that the coating has both a high gloss and is oxidizable, and thus printable. A single, dual purpose coating was not previously possible with polyethylene or polypropylene. The disclosed process and article of manufacture are based on a low density polyethylene of narrow molecular weight distribution having a melt index of 2.0 to 4.0 dg/min, a swell ratio of 1.2 to 1.35, an annealed density of 0.91 to 0.93-g/cc, and a polydispersity index of between about 6 and about 9.
Copending, commonly assigned Application Ser. No. 09/023,900 (not yet assigned; Eastman Chemical Co. Docket No. 70537) discloses a coating and process therefore, producing a polyethylene extrusion coating also having a gloss comparable to that exhibited by polypropylenes, and providing for a coating having high stress crack resistance and hot tack ("pop-open" resistance), having superior neck-in properties and providing for tougher seals. The disclosed process and article of manufacture are based on a low density polyethylene of narrow molecular weight distribution having a melt index of 0.5 to 4.0 dg/min, preferably about 0.5 to below 2.0 dg/min, a swell ratio of 1.2 to 1.35, an annealed density of 0.91 to 0.93-g/cc, and a polydispersity index between about 5.5 and about 9.
The polyethylene claimed in the aforementioned U.S. Patents and the Patent Application permitted extrusion coatings with a gloss comparable or even superior to that of polypropylene, with polypropylene having previously been considered the glossiest coating material in the extrusion coating industry. Gloss, or specular reflectance, of a surface is a measure of the light reflected by that surface as compared to that which would be reflected by a perfect mirror. A gloss value of 100 is 1/10th of the assigned value of 1,000 for a perfect mirror. Gloss is typically measured at 75.degree. and/or 60.degree. (from the vertical) in accordance with ASTM D2457. A typical extrusion coated polypropylene exhibits a 75.degree. Surface Gloss of about 86, while the aforementioned Patents and Applications disclose extrusion coated polyethylenes exhibiting a 75.degree. Surface Gloss as high as, for example, 90.
However, even higher gloss coatings are desired in some extrusion coating applications. The extrusion coating industries, and photographic and copier paper makers in particular, are continually requesting their polymer suppliers to develop and supply polyethylenes that produce glossier films and coatings. Therefore it would be very desirable to develop a polyethylene which would provide even glossier films and coatings than those currently available, without detracting from the film or coating transparency of the currently available polyethylenes.